Method of passivating stainless steels



Patented Dec. 4, 1945 METHOD OF P)! SSIVATING STAINLESS STEELS Herbert H. Uhlig. Schenectady, N. Y., asslgnor to General Electric Company, a corporation of New York No Drawing. Application September 2, 1942, Serial No. 457,045

2 Claims.

This invention relatesto a method of treating stainless steels, particularly stainless steels of the 18-8 variety. to improve the corrosion resistance and appearance thereof.

Prior to my invention it has been common practice to passivate stainless steels such as steels containing 18 per cent chromium and 8 percent nickel by immersion in a solution of nitric acid or a mixture of nitric and hydrofluoric acids. The resulting improvement in the appearance and corrosion resistance of such an alloy has been sufficiently good so that this additional final treatment of the surface is an accepted step in the commercial preparation of stainless steels. The passivation phenomenon was believed to include the formation of a protective oxide film and thus 'to require the use of nitric acid or an equivalent oxidizing acid or acid mixtures.

The present invention is based on my discovery that the passivationf of stainless steels is not unique to oxidizing acids such as nitric. acid. I have found that equally acceptable results may be obtained by treating the stainless steel at room temperatures with a solution of hydrochloric acid containing a small amount of a pickling inhibitor. The minimum concentration of the hydrochloric acid solution found to be useful for my purposes is about 3 normal. Preferably the concentration ranges between 6 and 12.5 norme], although the preferred acid strength of the treating bath depends to a certain extent upon the type and nature of inhibitor used and on the steel composition and structure. The treating bath may comprise either a solution of hydrochloric acid or a solution of a mixture of sulfuric acid and sodium chloride of equivalent hydrochloric acid concentration.

In accordance with my invention the stainless steel is first cleaned and pickled, using any of the well known pickling baths. For example, the steel may be degreased in hot benzene or a chicrinated hydrocarbon solvent and thereafter pickled for five minutes at 80 degrees C. in a pickling bath containing 25 volume percent concentrated hydrochloric acid and 25 volume percent concentrated sulphuric acid. The pickled steel is then passivated by immersion in a solution of hydrochloric acid of the desired normality maintained at about room temperature and containing a small amount, preferably less than 1 percent of a suitable pickling inhibitor. The time required for' thistreatment depends on a number of factors including the strength of the bath, condition of the steel, etc. I have found that from 12 to 24 hours immersion is ample in most cases. remove traces of the. passivating material. some types of commercial inhibitors it may be found necessary to scrub the surfaces to remove a brown surface layer consisting largely of an The passivated steel is washed to.

With 9 attached layer of inhibitor. No such surface layer is obtained when the inhibitor is quinolin ethiodide. B means of this passivating treatment. the steel is given a pleasing, bright, satin surface indicating that the alloy has been etched or otherwise acted upon in a manner wholly unlike the action of acid pickles which produce a matte surface.

Steels treated in accordance with my invention have been exposed to a 20 percent sodium chloride spray at room temperature for a period of one month without suffering any appreciable change in the surface appearance or weight. These results are comparable with the results obtained with steels passivated by treatment with nitric acid solutions.

Stainless steel passivated according to this process and subsequently exposed to the action of a ten percent solution FeCl3.6H20 in 0.05N HCl exhibited a corrosion rate of the same order of magnitude as steels passivated by means of nitric acid. As will be shown, the amount of corrosion as measured by the loss of weight of the treated steel depends to some extent upon the acid strength of the treating bath and the type of in.- hibitor used.

The following table sets forth the average weight losses in the specimens due to the various surface treatments and also the weight losses resulting from corrosion tests using a ferric chloride solution as the corrosive medium:

Weight losses in surface treatment and in ferric chloride corrosion tests Av. wt. loss Av. wt. loss in surface by gluing in Concentration of acid used treatment 10% eChGEhO for surface treatment mgJcmJ/day mgJcmJ/day (24-hour (4-hour immersion) exposure) 18-8 STEEL A 2.97 N 0.153 9. 42 6.03 N HCl+0.5 per cent com- 0.672 9. 30 N mercial inhibitor 3'. 3. 31 12. 50 N 15. 4 2. 92 2. 97 N 0. 086 2. 51 6.03 N HC1+0.5 per cent quino- 0. 111 l. 99 9. 30 N lin ethiodide 0. 201 1.73 12. 50 N 0. 830 2. 93 i3; t 8'83 i2 7 9.30 N 0. 005 0.73 i 11. 50 N 0.002 0.84 Control (pickled but not surfacetreated) 4.4

18-8 STEEL B 3.0 N 0.221 6. 46 6.0 N HCl-i-Oj per cent com- 0.580 3.56 9.0 N melcial inhibitor 2. 89 l. 60 12.4 N 8. 51 2. 24 3. 0 N 0. 179 1. 47 6.0 N HC1+0.5 per cent quino- 0.147 0. 92 9.0 N linethiodide. 0.108 0.77 12.6 N 0.322 0.77

Steel A used in these tests contained 18.58 percent chromium; 8.48 percent nickel. and .05 percent carbon. Steel B contained 18.57 percent chromium; 9.63 percent nickel, and 0.14 percent carbon. Specimens of these steels were cleaned and pickled as previously described before being passivated by treatment in the designated acid baths. The "commercial inhibitor" was Grasselli inhibitor No. 3 furnished by E. I. duPont de Nemours and Company, Wilmington, Delaware.

As is indicated by the results set forth in the above table, the preferred composition of the bath is dependent upon a number of factors. In general, theimprovement in resistance to pitting obtained by treatment with a solution of hydrochloric acidand quinolin ethiodide is comparable with that accomplished by treatment with either cold or hot nitric acid. Three specimens of steel A immersed in 30 percent HNOa at 54 C. for 30 minutes, this corresponding approximately to conditions of some commercial passivating treatments, lost an average of 1.98 mg./cm. /day in ferric chloride solution as compared with an average loss of 1.99 mg./cm. /day for the specimen treated in the 6 N hydrochloric acid-0.5 percent quinolin ethiodide solution.

The corrosion resistance of specimens surfacetreated with hydrochloric acid plus the designated commercial inhibitor improves rapidly with increase in the concentration of hydrochloric acid from 3 N to 9 N. A resistance to pitting better than that of the original pickled surface occurs in the case of the higher acid concentrations. Preferably, with this particular inhibitor the normality of the acid bath should be at least about 8 normal. 3

From the above results it'will be seen that I have provided a new procedure for rendering stainless steels more corrosion-resistant, particularly insofar as their susceptibility to pit corrosion is concerned. The process is relatively simple and may be effected with a minimum of operations and by the use of readily available materials.

' What I claimas new and desire to secure by Letters Patent of the United States is:

1. The method of improving the appearance and resistance to corrosion of stainless steel containing about 18, percent chromium and about 8 percent nickel which comprises immersing said steel, after first being pickled, in anaqueous solution of hydrochloric acid having a normality of from 6 to 9 and containing less than one percent quinolin ethiodide.

2. The method of improving the appearance and resistance to corrosion of stainless steel containing about 18 percent chromium and about 8 percent nickel which comprises pickling said steel and immersing the pickled steel in an aqueous solution of hydrochloric acid containing a fractional percent of quinolin ethiodide, the said solution containing between 6 and 9 mols hydrochloric acid per liter of solution.

HERBERT H. UHLIG. 

